Methods and systems for fabricating high quality superconducting tapes

ABSTRACT

An MOCVD system fabricates high quality superconductor tapes with variable thicknesses. The MOCVD system can include a gas flow chamber between two parallel channels in a housing. A substrate tape is heated and then passed through the MOCVD housing such that the gas flow is perpendicular to the tape&#39;s surface. Precursors are injected into the gas flow for deposition on the substrate tape. In this way, superconductor tapes can be fabricated with variable thicknesses, uniform precursor deposition, and high critical current densities.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/139,127, filed on Apr. 26, 2016, which is a divisional of U.S. patentapplication Ser. No. 14/208,818, filed on Mar. 13, 2014, issued on Feb.13, 2018 as U.S. Pat. No. 9,892,827, which claims priority to U.S.Provisional Application No. 61/801,478, filed on Mar. 15, 2013, all ofwhich are herein incorporated by reference in their entirety.

GOVERNMENT SPONSORSHIP

Advanced Research Projects Agency-Energy (ARPA-E), award DE-AR0000141

BACKGROUND

Several materials systems are being developed to solve the loomingproblems associated with energy generation, transmission, conversion,storage, and use. Superconductors are a unique system that provides asolution across a broad spectrum of energy problems. Superconductorsenable high efficiencies in generators, power transmission cables,motors, transformers and energy storage. Further, superconductorstranscend applications beyond energy to medicine, particle physics,communications, and transportation.

Superconducting tapes are becoming more and more popular. This is inpart due to successful fabrication techniques that create epitaxial,single-crystal-like thin films on polycrystalline substrates (Y. Iijima,et al., “Biaxially Aligned YBa₂Cu₃O_(7-x) Thin Film Tapes,” Physica C185, 1959 (1991); X. D. Wu, et al., “Properties of YBa₂Cu₃O₇ Thick Filmson Flexible Buffered Metallic Substrates,” Appl. Phys. Lett. 67, 2397(1995); A. Goyal, et al., Appl. Phys. Lett. 69, 1795 (1996); V.Selvamanickam et al., “High Performance 2G wires: From R&D toPilot-scale Manufacturing,” IEEE Trans. Appl. Supercond. 19, 3225(2009)). Superconducting films that are processed by this techniqueexhibit critical current densities comparable to that achieved inepitaxial films grown on single crystal substrates. Using thistechnique, several institutions have demonstrated pilot-scalemanufacturing of superconducting composite tapes. One popular processused to manufacture superconducting tapes is called metal organicchemical vapor deposition (MOCVD) (V. Selvamanickam et al., “HighPerformance 2G wires: From R&D to Pilot-scale Manufacturing,” IEEETrans. Appl. Supercond. 19, 3225 (2009)).

Current MOCVD methods and systems used for manufacturing ofsuperconductor tapes have significant drawbacks, which are primarilyrooted in design flaws (V. Selvamanickam et al., “Method forManufacturing High-Temperature Superconducting Conductors,” U.S. Pat.No. 8,268,386). For example, FIG. 1 illustrates a schematic of theshowerhead 100 used in current MOCVD systems used for manufacturing ofsuperconductor tapes. The core of the MOCVD system is a reactor 110which consists of a showerhead 115 to disperse the precursor flow 120 ona tape 125 and a heater 130 to heat the tape 125 by contact heating asit travels along the heater 130.

One major drawback of the current MOCVD design is that the heating anddeposition mechanisms do not provide uniform heating or uniformdeposition on the tape. These design flaws produce superconductor tapeswith a poor surface microstructure, which can significantly deterioratethe tape's superconducting quality. For example, the superconductor tapeis heated by a fairly bulky heater by means of contact heating, and so aconstant heater temperature does not necessarily yield a constant tapetemperature, especially in thicker films. In addition, since the tapetravels quickly over the heater, there are sporadic losses of contactbetween the tape and heater. And because the tape has a very small mass,even brief losses of contact result in significant decreases in tapetemperature. Furthermore, precursor flow is directed downwards towardsthe tape, and once it hits the tape it flows sideways across the tape.This non-uniform flow causes temperature differences across the tape,and thus deviations from the optimum temperature window. All of thesetemperature fluctuations can cause the process to deviate from theoptimum temperature window and produce misoriented grain growth in thetape's surface microstructure. In addition, there are no solutionsavailable in current systems to directly monitor tape temperature sincethere is no line of sight available from outside the reactor, and thereis no room in the reactor to monitor the tape temperature directlywithout interfering with the precursor flow. Current MOCVD designsmonitor temperature using a thermocouple inside the heating block. Butbecause the tape temperature is not uniform deviations from the optimumtemperature window typically go unnoticed.

Furthermore, the deposition flow from the showerhead reaches the tapeonly at the center, and most of the other flow is pumped out withoutfully reaching the substrate. This non-uniform precursor flow results innon-uniform superconductor growth, including misoriented grain growth.This deposition phenomenon is illustrated in FIG. 2, which is a finiteelement analysis of turbulent fluid flow and solid/fluid heat transferin a current MOCVD system. The streamlines 200 show the flow path of theprecursor, while the color differences show flow inhomogeneity. Thefinite element plot illustrates that a substantial fraction of theprecursor (especially that injected away from the center of theshowerhead) does not make it to the tape surface, which reduces theconversion efficiency of precursor to film. Also, the non-uniform flowrate will cause non-uniform film deposition rate, which in turn cancause inhomogeneities in the film.

The aforementioned heating and deposition drawbacks are exacerbated asthe superconducting film is thickened during fabrication. Thus, as thefilm is thickened misoriented grain growth increases. For example, FIG.3A shows the surface microstructure of a 1 μm thick superconductor tape300 fabricated by current MOCVD methods. The microstructure is fairlyhomogenous with relatively little grain misorientation 310. However, asseen in FIG. 3B, a 2 μm thick superconductor tape 320 presents with asubstantial amount of misoriented grains 330. FIG. 3C shows a crosssection of the 2 μm tape 320 with a-axis grains 330. These illustrationsprove that the misoriented grains predominantly form after the initial 1μm of tape is fabricated.

As misoriented grain growth increases with tape thickness, criticalcurrent density (critical current/cross sectional area) decreases, i.e.the quality of the superconductor film degrades with increasing tapethickness. This phenomenon is illustrated in FIG. 4, which showscritical current density as a function of superconductor tape thicknessin tape made by a current MOCVD system. One explanation for this trendis that increases in misoriented grain growth in thicker films impedethe flow of current, thus resulting in lower current densities. And whenthe current density decreases, the quality of the superconducting tapedegrades. This indicates that the present-day MOCVD process is notsuitable to fabricate high-quality superconducting tapes thicker thanapproximately 1 μm.

Another drawback to current MOCVD design is that the process isinefficient and costly. For example, as explained above, the MOCVD'sdeposition design can result in non-uniform tapes where most of the flowis pumped out without fully reaching the substrate. In addition, theshowerhead is positioned at a substantial distance from the tape andheating block in order to avoid premature precursor decomposition fromthe heating block. As a result, expensive precursors are wasted in themanufacturing process leading to lower throughput. In addition, thedeposition process is relatively slow and results in low yields. Forexample, the current process uses 2,2,6,6-tetramethyl-3,5-heptanedionate(-thd) metal organic complexes of Y or other rare earth (RE), Ba, and Cuas a precursor. The -thd complexes are deposited on the substrate anddissociate at an appropriate temperature and oxygen partial pressure toform YBa₂Cu₃O_(x) superconductor (or REBa₂Cu₃O_(x) (REBCO, RE=rareearth)). In the present MOCVD system, precursor dissociation is entirelythermally activated by the high substrate temperature. But thermalactivation alone does not result in complete dissociation of precursors,and so, precursor to film conversion efficiency is low—only about 15% ofthe theoretical value.

Furthermore, it would be desirable to activate precursors using plasmaactivation. However, it is not feasible to introduce plasma in prior artreactors because the metallic showerhead and susceptor don't allow forit (V. Selvamanickam et al., “Ultraviolet (UV) and plasma assistedmetalorganic chemical vapor deposition (MOCVD) system,” U.S. Pat. Pub.No. 2004/0247779).

Thus, there is need in the art for methods and systems that canfabricate superconducting tapes having films with varied thicknesses atconstant temperature, with uniform precursor deposition, and in highyields and efficiencies (e.g., >15%). There is also need in the art formethods and systems that can fabricate superconducting tapes with filmsat higher thicknesses (e.g., up to 3 μm thick) with minimal misorienteda-axis grain growth and high critical current densities. Finally, thereis need in the art for a system that can fabricate superconductor tapesvia plasma activation.

SUMMARY

A metal organic chemical vapor deposition (MOCVD) system can be used tofabricate high quality superconductor tapes. In one arrangement, theMOCVD system described herein can include a gas flow path between twochannels. A substrate tape can be heated and then delivered to the MOCVDsystem perpendicular to the gas flow path. The gas flow may include oneor more precursors delivered parallel or perpendicular to the substratetape's plane. The precursor can decompose thermally as it contacts theheated substrate tape to deposit the superconductor film. The result maybe a high quality superconductor tape.

In one embodiment, the MOCVD system described herein can fabricate asuperconductor tape at a constant temperature with uniform precursordeposition. In another embodiment, the MOCVD system described herein canfabricate a superconductor tape with at least one or more of variablethickness, high critical current density, and minimal misoriented graingrowth. In yet another embodiment, the MOCVD system described herein canfabricate a superconductor tape with a high conversion efficiency ofprecursor to film.

In yet another embodiment, superconductor tapes made in accordance withthe apparatuses described herein comprise variable film thicknesses withat least one of high critical current densities, no grainmisorientation, and high precursor to film conversion efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description,will be better understood when read in conjunction with the appendeddrawings. For the purpose of illustration only, there is shown in thedrawings certain embodiments. It's understood, however, that theinventive concepts disclosed herein are not limited to the precisearrangements and instrumentalities shown in the figures.

FIG. 1 is a schematic of a showerhead used in prior art MOCVD systems.

FIG. 2 is a finite element analysis of turbulent fluid flow andsolid/fluid heat transfer in prior art MOCVD systems

FIGS. 3A-3C illustrate surface microstructures of 1 μm and 2 μm thicksuperconductor tapes fabricated by prior art MOCVD systems.

FIG. 4 illustrates critical current density as a function ofsuperconducting tape film thickness in tape fabricated by prior artMOCVD systems.

FIGS. 5A-5B illustrate schematics of an improved MOCVD system, inaccordance with an embodiment.

FIG. 6 is a temperature profile of a superconducting tape heated by animproved MOCVD system, in accordance with an embodiment.

FIG. 7 illustrates critical current density measurements of a 1.8 μmthick REBa₂Cu₃O₇ superconducting tape made by an improved MOCVD system,in accordance with an embodiment.

FIG. 8 illustrates a surface microstructure of a 2 μm thick REBa₂Cu₃O₇superconducting tape made by an improved MOCVD system, in accordancewith an embodiment.

FIGS. 9A-9C illustrate surface, cross sectional microstructures and 2-DX-ray diffraction data from a 3.2 μm thick REBa₂Cu₃O₇ superconductingtape made by an improved MOCVD system, in accordance with an embodiment.

FIGS. 10A-10B illustrate cross sectional microstructures of 1.3 and 4.1μm thick REBa₂Cu₃O₇ superconducting tapes made by an improved MOCVDsystem, in accordance with an embodiment.

DETAILED DESCRIPTION

Before explaining at least one embodiment in detail, it should beunderstood that the inventive concepts set forth herein are not limitedin their application to the construction details or componentarrangements set forth in the following description or illustrated inthe drawings. It should also be understood that the phraseology andterminology employed herein are merely for descriptive purposes andshould not be considered limiting.

It should further be understood that any one of the described featuresmay be used separately or in combination with other features. Otherinvented systems, methods, features, and advantages will be or becomeapparent to one with skill in the art upon examining the drawings andthe detailed description herein. It's intended that all such additionalsystems, methods, features, and advantages be protected by theaccompanying claims.

FIGS. 5A and 5B show an embodiment of an improved MOCVD system 500 and across-section of the improved MOCVD system 500, respectively. In oneembodiment, the system may primarily comprise an Ohmic heating system510, a housing 520, a flow chamber 530, an array of electrodes 540, anda groove 550. The improved MOCVD 500 can be designed to heat a substratetape 560 and buffer layer to an optimum temperature followed by aprecursor deposition to fabricate a superconducting film 570. Theimproved MOCVD 500 may be designed to fabricate tape 570 withsubstantially thick superconducting film with a high current density byapplying uniform heating and uniform precursor deposition to a substratetape 560. In one embodiment, the film thickness is between approximately1 μm and 10 μm.

In another embodiment, the substrate tape 560 may contact a surface of aroller 510 made of highly-conductive metal such as copper, outside thedeposition zone. The metal roller 510 may heat the tape 560 directlythough DC electric current flowing through the roller 510, i.e. Ohmicheating. In one embodiment, the tape is heated to approximately 700-800°C. Direct ohmic heating provides a means for obtaining highly uniformtemperature distribution across the length of the tape 560. Since theheating is not from an external heat source, the stability of the tape560 temperature can be maintained over long manufacturing processes.Furthermore, because the tape 560 is suspended and only in contact withthe rollers 510 at the tape 560 ends, the result is a substantiallysmall thermal mass compared to a prior art heating block. In additioncontact heating and the associated problems that exist in conventionalsystems (tape in contact with susceptor/heating block) are completelyeliminated. Also, since the heating is not by contact with an externalheater, sporadic fluctuations in temperature due to loss of contact canbe eliminated. Furthermore, in yet another embodiment, the current canbe flowed from the rolling heater 510 through a high resistive metallictape such as Hastelloy or Inconel, which may act as the substrate forthe superconductor tape 560 or a carrier for the tape 560.

In an embodiment, the substrate tape 560 may be designed at an optimalresistance for Ohmic heating. For example, the substrate tape 560 may bedesigned to have a high enough resistance so that a practical currentcan be applied to heat the tape 560. In addition, the tape 560 may alsobe designed to have a low enough resistance so that a practical voltagecan be applied to heat the tape 560. For example, in one embodiment, thesubstrate tape 560 can be designed to have a resistance of approximately1 ohm. A 1 ohm tape may require a power supply of 20V and 20 A, which isreadily available.

In an embodiment, the substrate tape 560 may travel through the MOCVDhousing 520 via a groove 550. As the substrate tape 560 travels throughthe groove 550 it may encounter one or more temperature monitoringdevices 580. In one embodiment, the temperature monitoring devices 580are optical crystal probes 580. In another embodiment, the opticalcrystal probes 580 are positioned beneath and in close proximity to thetraveling substrate tape 560 to monitor the temperature of the backsurface of the tape 560. The probes 580 can be positioned outside of theflow chamber 530 so as to not interfere with the gas flow duringfabrication. In one embodiment, the temperature signal collected by aprobe 580 is fed back to the heating system 510 in a closed PID loop tocontrol the power of the heater system's 510 DC power supply to maintaina constant temperature. Because the thermal mass of the tape 560 issubstantially small, fluctuations in temperature can be immediatelyrecognized by the optical probes 580 achieving tight temperaturecontrol. In yet another embodiment, the optical crystal probes 580 maybe placed in an array. FIG. 6, by way of example only, illustrates atape temperature profile collected by the optical probes 580 during tape570 fabrication in an improved MOCVD 500, in accordance with anembodiment. The tape temperature may be maintained within asubstantially narrow temperature range during fabrication.

Referring to FIG. 5B, as the substrate tape 560 travels through thegroove 550 it may be exposed to a gas flow in a flow chamber 530positioned in the interior of the housing 520. Gas may enter the housing520 through a gas inlet 585 and leave the housing through a gas outlet590. In one embodiment, the gas may include argon and oxygen. In anotherembodiment, as the gas enters the housing 520 it may pass through aninlet pressure buffer chamber 591, followed by a second inlet dispersionplate 592. In another embodiment, the gas may first pass through aninlet dispersion plate 593 before passing through the inlet pressurebuffer chamber 591. In one embodiment, the gas may pass through theinlet pressure buffer chamber 591, which can have a large volume. Then,the gas may pass through the inlet dispersion plate 592 that can includeone or more substantially small holes. The inlet dispersion plate 592may have a small conductance and can uniformly distribute the gasthroughout the flow chamber 530. This inlet design can alsosubstantially absorb upstream pressure fluctuations (e.g., fluctuationscaused by the precursor delivery system). As a result, pressurefluctuations can be minimized at the substrate tape 560 site so that thetape 560 is fabricated at a substantially constant and optimumtemperature.

In another embodiment, as the gas exits the housing 520 it may passthrough an outlet dispersion plate 594 followed by an outlet pressurebuffer chamber 595. In yet another embodiment, the gas may pass througha second outlet dispersion plate 596 after passing through the outletpressure buffer chamber 595. In one embodiment, the gas may pass throughan outlet dispersion plate 594 that can include one or moresubstantially small holes. Then, the gas may pass through the outletpressure buffer chamber 595, which can have a large volume. This outletdesign can substantially absorb downstream pressure fluctuations causedby the downstream pump and valves. Again, as a result, pressurefluctuations can be minimized at the substrate tape 560 site so that thetape 560 is fabricated at a substantially constant and optimumtemperature.

In one embodiment, the gas may include one or more precursors. Theprecursors may flow through the chamber 530 to contact the tape 560 fordeposition. In another embodiment, the precursor is controlled at amoderate vacuum approximating 2 Torr. In yet another embodiment, the oneor more precursors flow through the chamber 530 at a direction parallelto the tape's 560 plane. In still another embodiment, the one or moreprecursors flow through the chamber 530 at a direction perpendicular tothe tape's 560 long axis (cross-flow).

In one embodiment, before the precursor enters the flow chamber 530, itcan be vaporized from liquid to vapor when mixed with a hot argoncarrier gas. In another embodiment, oxygen gas is injected into theprecursor flow to assist the reaction kinetics. In yet anotherembodiment, once vaporized, the precursor is injected into the improvedMOCVD system 500. The flow channel 530 can be maintained at atemperature between approximately 250-300° C. to prevent the precursorfrom condensing on the channel walls. Because the tape 560 is heated toa substantially higher temperature (e.g., ˜700-750° C.), the precursordecomposes thermally as it contacts the tape 560 thus depositing thesuperconductor film.

The aforementioned temperature control design allows for a substantialreduction in the process zone size. For example, the flow of precursormay be directed between two parallel channels 597 that form asubstantially small gap 598. The precursor can then flow perpendicularto the tape 560. With such a small gap 598, the flow channel 530 may behighly uniform and laminar with a low volume and low profile. Such flowconditions can eliminate turbulence, temperature losses due toconvective heat transfer, and any non-uniformity in flow. Furthermore,the design can maximize conversion efficiency from vapor phase to tape560 since the flow is confined to a small volume substantially near thetape 560. In one embodiment, the gap 598 between the parallel channels597 may be modified to a certain size to achieve a desired performancelevel. For example, the size of the gap 598 may vary from less than 1 mmto approximately ¼ inch.

In an embodiment, the flow chamber 530 further includes an array of oneor more parallel-plate capacitive electrodes 540 for plasma activation.It's understood that the dimensions of the electrode 540 (i.e. the sizeof the plasma) can be flexible and customized to achieve a desiredperformance level. The electrodes 540 may be positioned in the vicinityof the precursor flow, the vicinity of the tape 560, or both. Duringfabrication, the electrodes 540 can be connected to a plasma source.Plasma can be introduced in the flow channel 530 to activate theprecursors. This plasma activation can improve reaction kinetics andthus further increase the precursor disassociation rate from the vaporphase to the tape 560. This design can substantially improve theprecursor to film conversion efficiency. In one embodiment, the plasmacan be introduced upstream of the tape 560. In another embodiment, theplasma can be introduced directly over the tape 560.

It's understood that the above description is intended to beillustrative, and not restrictive. The material has been presented toenable any person skilled in the art to make and use the inventiveconcepts described herein, and is provided in the context of particularembodiments, variations of which will be readily apparent to thoseskilled in the art (e.g., some of the disclosed embodiments may be usedin combination with each other). Many other embodiments will be apparentto those of skill in the art upon reviewing the above description. Thescope of the invention therefore should be determined with reference tothe appended claims, along with the full scope of equivalents to whichsuch claims are entitled. In the appended claims, the terms “including”and “in which” are used as the plain-English equivalents of therespective terms “comprising” and “wherein.”

EXAMPLES Example 1

FIG. 7, by way of example only, displays the current-voltagecharacteristics (i.e. critical current density) of a 1.8 μm thickREBa₂Cu₃O₇ film. The film was fabricated in an improved MOCVD system,such as the embodiment described in FIGS. 5A-5B. The data shows that thetape has a critical current at 916 A over a 12 mm width, at 77 K in azero applied magnetic field, which corresponds to a current density at4.24 MA/cm². This is a substantially high current density for anMOCVD-fabricated superconductor tape with thickness greater than 1 μm.

Example 2

FIG. 8 illustrates the surface microstructure of a 2 μm thick REBa₂Cu₃O₇film. The film was fabricated in an improved MOCVD system, such as theembodiment described in FIGS. 5A-5B. The tape is substantially uniformwith no observable misoriented grain growth.

Example 3

FIG. 9A illustrates a surface microstructure of a 3.2 μm thickREBa₂Cu₃O₇ film. The film was fabricated in an improved MOCVD system,such as the embodiment described in FIGS. 5A-5B. The tape issubstantially uniform with no observable misoriented grain growth. Thereare some secondary phases relegated to the surface. FIG. 9B shows across section microstructure of the same 3.2 μm thick REBa₂Cu₃O₇ film.The cross-section is substantially homogenous and also shows nomisoriented grain growth. FIG. 9C shows 2-D X-ray Diffraction data forthe 3.2 μm thick REBa₂Cu₃O₇ film. Again, this data shows that the filmlacks misoriented grain growth (i.e. no diffraction spots correspondingto non (001) orientation, perpendicular to the central horizontal axis).The data further reveals the presence of grains only along the c-axis(i.e. diffraction spots along the central horizontal axis).

Example 4

FIG. 10A illustrates a Scanning Electron Microscopy (SEM) micrograph ofa cross-section of a 1.3 μm thick REBa₂Cu₃O₇ film (made by Focused IonBeam milling). The film was fabricated in an improved MOCVD system, suchas the embodiment described in FIGS. 5A-5B. The film was fabricated at achannel width of ¼″, flow rate of 2.5 mL/min, 0.05M/L precursormolarity, and 2.1 cm/min deposition speed. When these identicalconditions are applied in a conventional MOCVD system, the result is aREBa₂Cu₃O₇ film with 0.85 μm thickness. In other words, the improvedMOCVD system herein provides a 153% improved precursor conversionefficiency over conventional systems. Similarly, FIG. 10B illustrates aScanning Electron Microscopy (SEM) micrograph of a cross-section of a4.1 μm thick REBa₂Cu₃O₇ film (made by Focused Ion Beam milling). Thefilm was fabricated in an improved MOCVD system, such as the embodimentdescribed in FIGS. 5A-5B. The film was fabricated at a channel width of⅛″, flow rate of 2.5 mL/min, 0.1M precursor molarity, and 2.1 cm/mindeposition speed. When these identical conditions are applied in aconventional MOCVD system, the result is a REBa₂Cu₃O₇ film with 1.7 μmthickness. In other words, the improved MOCVD system herein provides a240% improved precursor conversion efficiency over conventional systems.

What is claimed is:
 1. A method of fabricating a superconductor tape bymetal organic chemical vapor deposition using a precursor to filmconversion efficiency greater than approximately 20% of theoreticalvalue, wherein the precursor to film conversion efficiency is a resultof a conversion of precursor to film performed within a gap in a rangefrom less than 1 mm to approximately ¼ inch, and wherein the gap isdefined by an upper surface and a lower surface parallel to the uppersurface.
 2. The method of claim 1, wherein the superconductor tape is aREBCO film.
 3. The method of claim 2, wherein the REBCO film comprisesthe formula REBa₂Cu₃O₇.
 4. The method of claim 1, wherein thesuperconductor tape is substantially uniform.
 5. The method of claim 1,wherein the superconductor tape comprises no observable misorientedgrain growth.
 6. The method of claim 1, wherein the superconductor tapecomprises a film thickness greater than approximately 2 μm and acritical current density greater than approximately 4 MA/cm² at 77 K ina zero applied magnetic field.
 7. The method of claim 6, wherein thesuperconductor tape further comprises non c-axis grain orientation lessthan approximately 10%.